1. Field of the Invention
The invention relates to condition responsive control circuits, and in particular, to circuits for controlling the application of electrical power to a unit, such as a electrical motor.
2. Description of the Prior Art
The prior art, as exemplified in U.S. Pat. Nos. 2,945,133, 3,366,843, 3,404,313, 3,416,060, 3,526,809, 3,555,356, 3,648,074, and 3,712,991, contains many control circuits which are responsive to a condition such as temperature. Some of the prior art motor control circuits are designed for interrupting the flow of electrical current to a motor if a sensed temperature of the motor windings exceeds a selected level; several of these motor control circuits have provisions for changing the temperature sensing level of the control circuit when the motor is running or when an excessive current condition indicating a locked rotor is sensed. Generally the prior art control circuits are characterized as having one or more deficiencies such as being unduly complex, requiring expensive specially manufactured components, having excessive labor costs in assembling, or having excessive variation in response with changes in ambient temperature.
Other systems used for motor protection include manual reset circuit breakers which act to rapidly respond to open the motor circuit under high amperage draw conditions. The manual reset requirement means that such devices will not restart until manually reset although in some cases the condition that caused the circuit to break may no longer exist after a period of time. Some control systems have a supplementary time delay device which prevents a try for a motor restart to be made until after a time delay from shutdown. This allows pressure bleed-off in refrigeration compressor applications to prevent starting stallouts. These systems are relatively costly in that both a circuit breaker and a time delay device are required.
Another motor protection device which provides protection from low voltage and higher than normal amperage draw conditions is a low voltage dropout relay which may be used to control the motor contactor. Under low voltage conditions the low voltage dropout relay acts to open the circuit to the contactor and protect the motor. They also prevent low voltage startup when stallout is likely. In this system the cost of the low voltage relay is in addition to the cost of the contactor.
For motor protection against insulation damage at high temperatures, thermal fuses and thermally tripped relays are available. Fuses are objectionable due to system shutdown until they are replaced. The conventional thermally tripped relay shuts off the motor at some temperature, as for example 121.degree.C and allows restart at some lower temperature such as 66.degree.C. Under slow temperature rise conditions such a system may be satisfactory. If a locked rotor condition exists, the rate of temperature rise is so rapid that the temperature can materially overshoot the temperature limit and cause motor winding insulation damage. A condition which causes subsequent recycling with continued lock rotor can further damage the motor from repeated temperature excusions into the insulation damaging range.